You won’t get pregnant if you use the pill, right? Wrong.

I used the pill for a decade before I began having children in my early 30s. It seemed like a foolproof method of birth control.

After having two children fairly close together, I felt that my family was complete. So imagine my surprise when I turned up pregnant in the spring of 2011 — while on the pill.

After some panicked Googling, I thought I had found the culprit: A few weeks earlier, I’d come down with strep, and the family doctor had prescribed an antibiotic. I had taken the antibiotic with no thought whatsoever about birth control.

Aha! That explained it, I thought. And the doctor later confirmed my suspicion: The antibiotic, she said, might have lowered the efficacy of the pill. (This turns out not to be the likely explanation — more on that below.) A week later, I had a miscarriage.

We changed insurance plans, and soon I found a highly recommended OB/GYN and asked about getting an intrauterine device. Based on some factors related to the ultrasound the OB required before ordering an IUD, she talked me out of it. Furthermore, an IUD would have cost me about $600, even with our excellent insurance plan.

Figuring that my previous unplanned pregnancy was just a fluke, I went back on the same pill. It cost me only $5 per month, I was a reliable pill taker, and switching to a different form of birth control seemed like a hassle. A few months later, I discovered I was pregnant again.

My third child is now 9 months old. I wouldn’t change anything and can’t imagine my life or our family without her.

Still, most women take the pill because they expect it to work. There’s something unsettling about using the most common form of birth control and then finding out that you have so little “control.”

The responses I got from local obstetricians ranged from: “Did you take the pill at about the same time every day?” (that, it turns out, shouldn’t have mattered) to wonky and wrong explanations about differences in formulation between generic and brand-name pills.

Getting to the bottom of why the pill failed me — not once, but twice — has felt about as scientific as voodoo. As I soon found out, there are more questions than answers about why the pill usually works but sometimes doesn’t.

The failure rate

Researchers are struggling to understand how a person’s individual traits — genetics, metabolism, obesity, diet and other factors — might make certain drugs, including the pill, work reliably for some people but not for all.

“I can’t even give you a scientific answer, even if you missed a pill,” says Colleen Krajewski, an OB/GYN who is affiliated with Johns Hopkins for contraception-related research projects, when asked to explain what might have caused my pill failures.

“My guess is it was sort of the perfect storm of little things,” she says.

The first thing that obstetricians like to remind you is that no method of birth control is 100 percent effective.

Researchers break statistics on efficacy of contraceptives into two categories: perfect use and typical use.

With perfect use — a term that basically applies to women in research studies who are being paid to take the pill in a regimented fashion — the pill is 99 percent effective. But with typical use, meaning women in the general population who use the pill imperfectly, the effective rate drops to 91 percent.

The gap comes down to adherence, says Princeton University demographer James Trussell, who has written extensively on contraceptive issues. “It’s not taking the pill. Or running out. There are a million reasons for this,” says Trussell.

Still, a review published in May in the journal Contraception shows that most unintended pregnancies on typical pills (those containing a combination of estrogen and progestin) occur only after two or three missed doses in a row.

“If a woman misses one pill every cycle, probably it’s not going to have a lot of impact on the pill’s effectiveness,” says Caroline Moreau, a Johns Hopkins epidemiologist specializing in fertility and contraceptive use. “Levels of hormones are going to be high enough that it’s going to suppress ovulation even if one pill is missed for 24 hours.”

An exception to that rule is progestin-only pills, which require taking the pill at a specific time of day.

But I had not missed a pill: My packs were clearly labeled by day, and it would have been easy to see if one had been skipped.

Was it metabolism?

So what other explanations might there be? Researchers are looking at how pills might perform differently depending on a person’s metabolism.

“We know that some people are fast metabolizers and some people are slow metabolizers,” says Alison Edelman, an associate professor of obstetrics and gynecology at the Oregon Health and Science University in Portland. A fast metabolizer might pass a drug through her liver too quickly; in the case of the pill, this could lower the hormone level below the critical threshold.

Was it obesity? Diet?

Another major question involves how obesity might affect the efficacy of the pill. Much of the research on the pill took place decades ago, prior to the obesity epidemic. In addition, clinical trials typically include the healthiest women and exclude those with health risks that might confound the studies.

“Obesity affects processes in the body that we cannot see or even fathom,” including how the body “sees” and responds to drugs, Edelman says. With the pill, obesity alters drug clearance, which could affect pill efficacy or increase the risk for failure when combined with a missed or late pill, she says.

Other potential culprits: Diarrhea or vomiting that causes foods and liquids to pass through the body rapidly, and changes in eating habits, such as adopting a grapefruit diet, which might alter absorption of the pill and affect hormone levels.

Contrary to what doctors often tell patients, common antibiotics such as amoxicillin and erythromycin have not been shown to interfere with the pill, Edelman says. Only “big gun” antibiotics such as rifampin — used to treat tuberculosis — are known to affect the efficacy of the pill.

I never did figure out why I got pregnant while on the pill. And that, researchers say, may just be how it is.

I’ll never know. I found peace with it and switched to an IUD. For good measure, my husband got a vasectomy, too.

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